KCheckbox description prop, simplified logic around slot vs label showing

[nucleogenesis]

Description

Issue addressed

Fixes #35

I also thought that the v-if-else block there was weird because if I gave both a slot and a label it would show neither - so I picked one to be the override.

Now if you give a slot and a label you'll see what is passed to the slot - if you don't give a slot it's expected you give a label.

Before/after screenshots

Before

After

Steps to test

1. yarn link kolibri-design-system in your kolibri local repo
2. Checkout this PR in kolibri-design-system repo
3. Run Kolibri dev server
4. Find an instance of KCheckbox in the code and find it in the app itself
5. Add a description prop with a value.

At a high level, how did you implement this?

Added the prop and updated the template to display it properly - basically stole the whole thing from KRadioButton.

Does this introduce any tech-debt items?

Nope.

Reviewer guidance

• Is the code clean and well-commented?
• Are there tests for this change?
• Are all UI components LTR and RTL compliant (if applicable)?
• Add other things to check for here

Comments

[sairina]

LGTM! Manually tested.